Mixtures of various triblock polyester polyethylene glycol copolymers having improved gel properties

ABSTRACT

A water soluble, biodegradable reverse thermal gelation system comprising a mixture of at least two types of tri-block copolymer components is disclosed. The tri-block copolymer components are made of a hydrophobic biodegradable polyester A-polymer block and a hydrophilic polyethylene glycol B-polymer block. The drug release and gel matrix erosion rates of the biodegradable reverse thermal gelation system may be modulated by various parameters such as the hydrophobic/hydrophilic component contents, polymer block concentrations, molecular weights and gelation temperatures, and weight ratios of the tri-block copolymer components in the mixture.

[0001] The application is a divisional and entitled the priority date ofU.S. patent application Ser. No. 09/559, 799, filed on Apr. 27, 2000.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to water soluble mixtures of two ormore types of biodegradable block copolymers and their use inparenteral, ocular, transdermal, vaginal, buccal, transmucosal,pulmonary, transurethral, rectal, nasal, oral, or aural administrationof drugs. More particularly, this invention relates to compositionshaving improved reverse thermal gelation properties comprising two ormore biodegradable triblock copolymers consisting of biodegradablepolyester and polyethylene glycol(PEG) blocks.

[0004] 2. Related Art

[0005] Many biologically active macro-molecules, such aspeptides/proteins and DNA, that are effective for gene therapy and avariety of therapeutic applications, have become commercially availablethrough advances in recombinant DNA and other technologies. However,these molecules are limited to parenteral administration due to theirsusceptibility to degradation in the gastrointestinal tract. Treatmentfor chronic illnesses or indications may require multiple injections perday over many days, or months. Patient compliance is usually poor.Therefore, it would be highly desirable to develop a system for thedelivery of bioactive agents or drugs, in particular, polypeptide orprotein drugs, at a controlled rate over a sustained period of timewithout the above mentioned problems. This system would help to optimizethe therapeutic efficacy, minimize the side effects, and thereby improvepatient compliance.

[0006] Drug loaded polymeric devices and dosage forms have beeninvestigated for long term therapeutic treatment of different diseases.Because of strict regulatory compliance requirements such asbiocompatibility, having a clearly defined degradation pathway, andsafety of the degradation products, there are currently few synthetic ornatural polymeric materials which can be used for the controlleddelivery of drugs, including peptide and protein drugs.

[0007] In copending U.S. application Ser. No. 09/396,589 filed Sep. 15,1999 which is a continuation in-part of U.S. application Ser. No.09/164,865 filed Oct. 1, 1998 which in turn is a continuation-in-part ofU.S. Pat. No. 6,004,573 which was filed on Oct. 3, 1997 and issued Dec.21, 1999, there is disclosed a biodegradable reverse thermal gelationsystem comprising ABA- or BAB-type block copolymers having an averagemolecular weight of between about 2000 and 4990 consisting of about 51to 83% by weight of a hydrophobic A polymer block comprising abiodegradable polyester, and about 17 to 49% by weight of a hydrophilicB polymer block consisting of polyethylene glycol(PEG). It is surprisingthat a block copolymer with such a large proportion of hydrophobiccomponent would be water soluble below normal room temperatures, such astemperatures as low as 5° C.

[0008] The reverse thermal gelation system referenced in the precedingparagraph causes minimal toxicity and minimal mechanical irritation tothe surrounding tissue because of the biocompatibility of the materialsand pliability of the gel. In addition, these block copolymersbiodegrade into non-toxic units. The drug release, gel strength,gelation temperature and degradation rate of each reverse gelationsystem can be controlled by proper design and preparation of the variouscopolymer blocks, namely, through modifications of the weight percent ofA-blocks and B-blocks, the mole percentages of lactate and glycolatemoieties making up the A-blocks, and the molecular weight andpolydispersity of the ABA or BAB triblock copolymers. Drug release isalso controllable through adjustment of the concentration of the blockcopolymer in the drug delivery liquid. It would be desirable to providea biodegradable reverse gelation system having a gelation temperaturewithin a desired range so that the system remains as a liquid at anambient temperature, but become a gel at the body temperature of theobject to which the drug is delivered. A reverse thermal gelation systemwith such gelation temperatures can be processed, formulated anddispensed at ambient temperatures, thereby significantly reducingmanufacturing and handling costs. In addition, accidental gelationduring application, e.g. gelation in the syringe during injection can beavoided. As discussed above, the gelation temperature of a reversethermal gelation system may be modified by changing the chain length,the glycolide/lactide(G/L) mole ratio of the A-polymer block, themolecular weight of the B-polymer block, the weight ratio of A block andB block polymers, and by various additives. However, the abovemodifications also change the gel qualification as well as the gelationtemperature. In addition, some additives may not be compatible with thedrug to be delivered. Therefore, it is desirable to provide a reversegelation system with adjustable gelation temperatures without changingits desirable gel qualities significantly. It has been discovered thatmixtures or blends of two or more tri-block polyester/polyethyleneglycol(PEG)copolymers provides for improved reverse thermal gelationproperties, such as an optimum gelation temperature, gel strength,degradation rate, and yet still maintains the desirable gel qualities.In addition, a combination of two or more different tri-blockpolyester/polyethylene glycol copolymers increases design flexibility.Such drug delivery systems have properties such as modulated drugrelease and modulated matrix erosion. These mixtures or blends oftri-block polyester\polyethylene glycol copolymers are very useful indrug delivery practice because they allow optimization to fit individualdrug or patient's needs. Therefore, the present invention provides for asignificant improvement in the art.

SUMMARY OF THE INVENTION

[0009] It is an object of the present invention to provide drug deliverysystems that are biodegradable, exhibit improved reverse thermalgelation behavior, and provide improved drug release characteristics.

[0010] A further object of this invention is to provide a drug deliverysystem for the parenteral administration of hydrophilic or hydrophobicdrugs, peptide or protein drugs, hormones, genes, oligonucleotides oranti-cancer agents.

[0011] Another object of this invention is to provide a method for theparenteral administration of drugs in a biodegradable polymeric matrixwhich results in the formation of a gel depot within the body from whichthe drugs are released at a controlled rate.

[0012] These and other objects are accomplished by providing abiodegradable copolymer composition having improved reverse thermalgelation properties, said composition comprising a mixture or blend ofat least two types of biodegradable polyester(A-block)/polyethyleneglycol(B-block) ABA or BAB tri-block copolymer components in a properratio so that the mixture has the desired reverse thermal gelationproperties. One component(Component I) of the triblock copolymer mixturehas an average molecular weight of between about 2500 and about 8000,preferably 3000 to 6500, and consists of a biodegradable polyesterA-polymer block and a polyethylene glycol(PEG) B-polymer block in aratio by weight of 1.3 to 3.0, preferably 1.8 to 2.6. The othercomponent(Component II) of the triblock copolymer has an averagemolecular weight of between about 800 and about 7200, preferably 1500 to6000, and consists of a biodegradable polyester A-polymer block and apolyethylene glycol(PEG) B-polymer block in a ratio by weight of 0.37 to2.6, preferably 0.54 to 2.5.

[0013] Preferably, the biodegradable polyester is synthesized frommonomers selected from the group consisting of D,L-lactide,D-lactide,L-lactide, D,L-lactic acid, D-lactic acid, L-lactic acid, glycolide,glycolic acid, -caprolactone, -hydroxy hexonoic acid, -butyrolactone,-hydroxy butyric acid, -valerolactone, -hydroxy valeric acid,hydroxybutyric acids, malic acid, and copolymers thereof. Morepreferably, the biodegradable polyester is synthesized from monomersselected from the group consisting of D,L-lactide, D-lactide, L-lactide,D,L-lactic acid, D-lactic acid, L-lactic acid, glycolide, glycolic acid,-caprolactone, -hydroxy hexanoic acid, and copolymers thereof. Mostpreferably, the biodegradable polyester is synthesized from monomersselected from the group consisting of D,L-lactide, D-lactide, L-lactide,D,L-lactic acid, D-lactic acid, L-lactic acid, glycolide, glycolic acid,and copolymers thereof. For purpose of illustration, the A-blockcopolymers are generally lactide or lactide-co-glycolide moieties.However, unless specifically referred to otherwise, the terms “lactide”,“lactate”, or “L” shall include all lactic acid derivatives and“glycolide”, “glycolate”, or “G” shall include all glycolic acidderivatives.

[0014] In the hydrophobic polyester A-block of both Components I and II,the molar ratio of lactate content to glycolate content (L:G ratio) isbetween about 3:1 and about 1:0, preferably between about 1:1 and about1:0.

[0015] Polyethylene glycol (PEG) is sometimes also referred to aspoly(ethylene oxide) (PEO) or poly(oxyethylene). The terms can be usedinterchangeably for the purposes of this invention. The averagemolecular weight of PEG in Component I is in a range of 900 to 2000,preferably 1000 to 1450. The average molecular weight of PEG inComponent II is in a range of 600 to 2000, preferably 900 to 1450.Preferably, Component I has a lower gelation temperature than ComponentII.

[0016] Mixing of two or more types of ABA or BAB triblock polyesterpolyethylene glycol copolymers can be done by mixing two or moreindividually synthesized tri-block copolymers, or by synthesizing two ormore tri-block copolymers in one reaction vessel. The mixture ofcopolymers resulting from the above two processes may have the same ordifferent gelation properties. In the latter method, both polymers havepolyester A blocks with the same lactide/glycolide ratio, molecularweight and polydispersity.

[0017] Additional objects and advantages of this invention will becomeapparent from the following detailed description of the variousembodiments and examples.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The above and other objects, features and advantages of theinvention will become apparent from the following detailed descriptionpresented in connection with the accompanying drawings in which:

[0019]FIG. 1 Corresponding to Example 3, illustrates the gelationtemperature of a solution (23% w/w) containing a mixture of Components Iand II as a function of percentage by weight of Component II in themixture.

[0020]FIG. 2 Corresponding to Example 5, illustrates the gelationtemperature of a mixture of a nearly water-insoluble copolymer(Component I), and a low molecular weight water-soluble copolymer(Component II) at varying concentrations.

[0021]FIG. 3 Corresponding to Example 6, illustrates the dissolution ofzinc-insulin from a gel composed of mixtures of Component I triblockcopolymers and Component II triblock copolymers in varying ratios.

[0022]FIG. 4 Corresponding to Example 7, illustrates the erosion (37°C.) of gels prepared by mixing two triblock copolymers with variousweight ratios of Component I and Component II.

DETAILED DESCRIPTION

[0023] This invention provides a reverse gelation system having improvedreverse thermal gelation properties which comprises mixtures of variousbiodegradable polyester/polyethylene glycol triblock copolymercomponents. The individual triblock copolymer components can besynthesized separately and then mixed, or be synthesized bypolymerization of two or more polyethylene glycol polymers havingdifferent molecular weights in one reaction vessel.

[0024] As used herein the following terms shall have the assignedmeanings:

[0025] “Parenteral” shall mean intramuscular, intraperitoneal,intra-abdominal, subcutaneous, and, to the extent feasible, intravenousand intraarterial.

[0026] “Gelation temperature” means the temperature at which thebiodegradable block copolymer undergoes reverse thermal gelation, i.e.the temperature below which the block copolymer is soluble in water andabove which the block copolymer undergoes phase transition to increasein viscosity to form a semi-solid gel. This temperature is also known asthe lower critical solution temperature(LCST).

[0027] The terms “gelation temperature” and “reverse thermal gelationtemperature”, “gel/sol temperature” or the like, shall be usedinterchangeably when referring to the gelation temperature.

[0028] “Polymer solution”, “aqueous solution”, and the like, when usedin reference to a biodegradable block copolymer contained in suchsolution, shall mean a water based solution having such block copolymermixtures or blends dissolved therein at a functional concentration, andmaintained at a temperature below the gelation temperature of the blockcopolymer mixtures.

[0029] “Reverse thermal gelation” is the phenomenon whereby a solutionof a block copolymer spontaneously increases in viscosity, and in manyinstances transforms into a semisolid gel, as the temperature of thesolution is increased above the gelation temperature of the copolymer.For the purpose of the invention, the term “gel” includes both thesemisolid gel state and the high viscosity state that exists above thegelation temperature. When cooled below the gelation temperature the gelspontaneously reverses to reform the lower viscosity solution. Thiscycling between the solution and the gel may be repeated indefinitelybecause the sol/gel transition does not involve any change in thechemical composition of the polymer system. All interactions to createthe gel are physical in nature and do not involve the formation orbreaking of covalent bonds.

[0030] “Drug delivery liquid” or “drug delivery liquid having reversethermal gelation properties” shall mean a polymer solution that containsa drug and which is suitable for administration to a warm-blooded animaland forms a gelled drug depot when the temperature is raised to or abovethe gelation temperature of the block copolymer.

[0031] “Depot” means a liquid drug delivery system which forms a gelupon the temperature being raised to or above the gelation temperaturefollowing administration to a warm-blooded animal.

[0032] “Gel” means the semi-solid phase that spontaneously occurs as thetemperature of the “polymer solution” or “drug delivery liquid” israised to or above the gelation temperature of the block copolymermixture.

[0033] “Aqueous polymer composition” means either a drug delivery liquidor a gel comprised of the water phase, having uniformly containedtherein, a drug and the mixture of biodegradable block copolymers. Attemperatures below the gelation temperature the mixture of blockcopolymers may be soluble in the water phase and the composition will bea solution. At temperatures at or above the gelation temperature, themixture of block copolymers will solidify to form a gel with the waterphase, and the composition will be a gel or semi-solid.

[0034] “Biodegradable” means that the block copolymer components canchemically break down or degrade within the body to form nontoxiccomponents. The rate of degradation can be the same or different fromthe rate of drug release.

[0035] “Drug” shall mean any organic or inorganic compound or substancehaving bioactivity and adapted or used for therapeutic purposes.Proteins, hormones, anti-cancer agents, oligonucleotides, DNA, RNA andgene therapies are included under the broader definition of “drug”.

[0036] “Peptide,” “polypeptide,” “oligopeptide” and “protein” shall beused interchangeably when referring to peptide or protein drugs andshall not be limited as to any particular molecular weight, peptidesequence or length, field of bioactivity or therapeutic use, unlessspecifically stated.

[0037] “Poly(lactide-co-glycolide)” or “PLG” shall mean a copolymerderived from the condensation copolymerization of lactic acid andglycolic acid, or, by the ring opening polymerization of -hydroxy acidprecursors, such as lactide or glycolide. The terms “lactide,”“lactate,” “glycolide” and “glycolate” are used interchangeably as notedabove.

[0038] “Polylactide” or “PLA” shall mean a polymer derived from thecondensation of lactic acid or by the ring opening polymerization oflactide. The terms “lactide” and “lactate” are used interchangeably.

[0039] “Biodegradable polyesters” refers to any biodegradablepolyesters, which are preferably synthesized from monomers selected fromthe group consisting of D,L-lactide, D-lactide, L-lactide), D,L-lacticacid, D-lactic acid, L-lactic acid, glycolide, glycolic acid,-caprolactone, -hydroxy hexanoic acid, -butyrolactone, -hydroxybutyricacid, -valerolactone, -hydroxy valeric acid, hydroxybutyric acids, malicacid, and copolymers thereof.

[0040] “Reverse thermal gel” or “reverse thermal gelation system” refersto all biodegradable triblock polyester PEG copolymers having reversethermal gelation properties. A detailed description of reverse gelationsystems has been disclosed and referenced above.

[0041] “Gel mixture” or “mixture of triblock copolymers” refers to areverse gelation system comprising two or more ABA or BAB triblockpolyester-polyethylene glycol copolymer components. The mixture can bemade either by simply mixing two or more individually synthesizedtriblock copolymer components, or by synthesizing two or more types ofcopolymer systems in one synthesizing vessel. The mixture prepared bythe above two processes may have the same or different gelationproperties and gel qualities.

[0042] The present invention is based on the discovery that mixtures oftwo or more triblock polyester PEG copolymers will provide for acopolymer mixture having improved reverse gelation properties such asflexible gelation temperature, gel strength, and yet still maintaindesirable gel qualities. In addition, such drug delivery systems havethe ability in order to modulate drug release, and matrix erosion. Thus,the combination of two or more individual triblock copolymerssubstantially increases design flexibility of the drug delivery systemto fit individual needs.

[0043] In a preferred embodiment, as illustrated in Table 1, theindividual component I triblock copolymers of this invention comprisetri-block copolymers of a biodegradable polyester and polyethyleneglycol(PEG) having an average molecular weight of between about 2500Daltons and about 8000 Daltons, preferably 3000 Daltons to 6500 Daltons,and consists of a biodegradable polyester block and a polyethyleneglycol(PEG) block in a ratio by weight of 1.3 to 3.0, and preferablybetween 1.8 to 2.6. The component II triblock copolymers have an averagemolecular weight of about 800 Daltons to about 7200 Daltons, preferably1500 Daltons to 6000 Daltons, and consists of a biodegradable polyesterand a polyethylene glycol(PEG) in a ratio by weight of 0.37 to 2.6,preferably between 0.54 and 2.5. The biodegradable, hydrophobic A-blockpolymer may be comprised of a polyester synthesized from monomersselected from the group consisting of D,L-lactide, D-lactide, L-lactide,D,L-lactic acid, D-lactic acid, L-lactic acid, glycolide, glycolic acid,-caprolactone, -hydroxy hexonoic acid, -butyrolactone, -hydroxy butyricacid, -valerolactone, -hydroxy valeric acid, hydroxybutyric acids, malicacid, and copolymers thereof. TABLE 1 Range Preferred Range Component I:B-polymer block MW  900 to 2000 1000 to 1450 Daltons DaltonsA-block/B-block weight 1.3 to 3.0 1.8 to 2.6 ratio L:G mole ratio inA-block 3:1 to 1:0 1:1 to 1:0 Tri-block copolymer MW 2500 to 8000 3000to 6500 Daltons Daltons Component II: B-polymer block MW  600 to 2000 900 to 1450 Daltons Daltons A-block/B-block weight 0.37 to 2.6  0.54 to2.6  ratio L:G mole ratio in A-block 3:1 to 1:0 1:1 to 1:0 Tri-blockcopolymer MW  800 to 7200 1500 to 6000 Daltons Daltons

[0044] The triblock copolymer components of the present invention may besynthesized by ring opening polymerization or by condensationpolymerization according to reaction schemes disclosed in U.S. Pat. Nos.5,702,717 and 6,006,573, hereby fully incorporated by reference. Forexample, the PEG blocks may be coupled to the biodegradable polyester byester or urethane links and the like. Condensation polymerization andring opening polymerization procedures may be utilized as may thecoupling of a monofunctional hydrophilic B block to either end of adifunctional hydrophobic A block in the presence of coupling agents,such as isocyanates. Furthermore, coupling reactions may followactivation of functional groups with activating agents such as carbonyldiimidazole, succinic anhydride, N-hydroxy succinimide and p-nitrophenylchloroformate and the like.

[0045] The hydrophilic B-block of the present invention is selected fromappropriate molecular weights of PEG. PEG was chosen as the hydrophilic,water-soluble block because of its unique biocompatibility, nontoxicity,hydrophilicity, solubilization properties, and rapid clearance from apatient's body.

[0046] The hydrophobic polyester blocks are selected because of theirbiodegradable, biocompatible, and solubilizing properties. The in vitroand in vivo degradation of these hydrophobic, biodegradable polyesterblocks is well understood and the degradation products are naturallyoccurring compounds that are readily metabolized and/or eliminated bythe patient's body.

[0047] Surprisingly, by blending or mixing two or more polyester PEGtriblock copolymers, either as individually synthesized copolymers orsynthesized together in one reaction vessel, the resulting triblockcopolymer mixture maintains a combination of desirable water solubilityand reverse thermal gelation properties. It is unexpected that, byselecting suitable triblock copolymer components having differentgelation temperatures and then mixing them in proper ratios, theresulting triblock copolymer mixtures have a reverse gelationtemperatures that varies between of 0 and 50° C., variable erosionduration from one day to ten weeks, and yet maintains desirable gelqualities. Aside from optimizing the gelation properties and gelqualities of each copolymer component, mixing two or more biodegradabletriblock copolymer components offers significantly increased flexibilityin designing drug delivery systems as necessary to fit individual needs.

[0048] The concentration at which the block copolymers are soluble attemperatures below the gelation temperature may be considered as thefunctional concentration. Generally speaking, total block copolymerconcentrations of as low as 3% and up to about 50% by weight can be usedand still be functional. However, concentrations in the range of about 5to 40% are preferred and concentrations in the range of about 10 to 30%by weight are most preferred. In order to obtain a viable gel phasetransition with the copolymer, a certain minimum concentration, e.g. 3%by weight, is required. At the lower functional concentration ranges,the phase transition may result in the formation of a weak gel. Athigher concentrations, a stronger gel network is formed.

[0049] A drug loaded triblock copolymer mixture having improved reversegelation properties may be prepared in the same manner as the previouslyreferenced individual triblock copolymer drug deliver systems areprepared. An aqueous solution of the triblock copolymer mixture of thepresent invention, at a temperature below the gelation temperature,forms a drug delivery liquid where the drug may be either partially orcompletely dissolved. When the drug is partially dissolved, or when thedrug is essentially insoluble, the drug exists in a colloidal state,such as a suspension or emulsion. This drug delivery liquid is thenadministered parenterally, topically, transdermally, transmucosally,inhaled, or inserted into a cavity such as by ocular, vaginal,transurethral, rectal, nasal, oral, buccal, pulmonary or auraladministration, whereupon it will undergo reversible thermal gelationsince body temperature will be above the gelation temperature.

[0050] Due to the biocompatibility, the biodegradability and thepliability of the triblock copolymers of the present invention, thismixture of the triblock copolymer components will cause minimal toxicityand mechanical irritation to the surrounding tissues. The drug release,gel strength, gelation temperature and degradation rate can becontrolled by the proper design and selection of the various copolymercomponents, namely, through proper selection of individual copolymercomponents and ratios of individual copolymer components to prepare themixture. The mixture will present modified and/or combinations of thegelation properties and gel qualities of the individual copolymercomponents. Particularly, mixtures of various types of triblockcopolymer components in the proper ratios provides for a wide range ofreverse gelation temperatures, such as from 0° C. to 50° C. With theavailability of a gelation temperature close to room temperature butbelow body temperature, provided by the copolymer mixture of the presentinvention, the handling procedure is simplified and the cost reduced. Italso decreases the risk of premature gelation during application, i.e.gelation in the syringe. More significantly, unlike other means ofmodifying gelation temperature, the copolymer mixtures of this inventionmaintain the desirable gel qualities of the individual copolymercomponents and yet provides for more flexibility in designing a morespecific drug delivery system to fit individual needs. Drug release isalso controllable through adjustment of the concentration of the mixturein the drug delivery liquid.

[0051] A dosage form comprised of a solution of a mixture of two or morecopolymer components that contains either dissolved drug, or drug as asuspension or emulsion, is administered to the body of a patient. Due tothe reverse thermal gelation properties of the block copolymer mixtures,this formulation then spontaneously gels to form a drug depot as thetemperature of the formulation rises to the gelation temperature. Theonly limitation as to how much drug can be loaded into the formulationis one of functionality, namely, the drug load may be increased untilthe thermal gelation properties of the copolymer mixtures are adverselyaffected to an unacceptable degree, or until the properties of theformulation are adversely affected to such a degree as to makeadministration of the formulation unacceptably difficult. Generallyspeaking, it is anticipated that in most instances the drug will make upbetween about 0.0001 to 30% by weight of the formulation with ranges ofbetween about 0.0001 to 20% being referred. These ranges of drug loadingare not limiting to the invention. Provided functionality is maintained,drug loadings outside of these ranges may also fall within the scope ofthe invention.

[0052] A distinct advantage to the compositions of the subject of thisinvention lies in the ability of the triblock copolymer mixtures toincrease the solubility of many drug substances. The combination of thehydrophobic polyester block(s) and hydrophilic block(s) PEG of thecopolymer components of the mixture renders the block copolymercomponent surface-active in nature. In that regard, it functions much asa soap or surfactant in having both hydrophilic and hydrophobicproperties. This is particularly advantageous for the solubilization ofhydrophobic or poorly water soluble drugs, such as cyclosporin andpaclitaxel. What is surprising is the degree of drug solubilization ofmost, if not all, drugs since the major content of the block copolymeris the hydrophobic polyester block content. However, as alreadydiscussed, even though the hydrophobic polymer block(s) are the majorcontent, the block copolymer is water soluble and it has been found thatthere is an additional increase in drug solubility when combined in anaqueous phase of the block copolymer.

[0053] Another advantage of the composition of the present inventionlies in the ability of the triblock copolymer mixture to increase thechemical stability of many drug substances. Various mechanisms fordegradation of drugs, that lead to a drug's chemical degradation, areretarded when the drug is in the presence of the block copolymermixture. For example, paclitaxel and cyclosporin A are substantiallystabilized in the aqueous polymer composition of the present inventionrelative to certain aqueous solutions of these same drugs in thepresence of organic co-solvents. This stabilization effect on paclitaxeland cyclosporin A is but illustrative of the effect that would beachieved with many other drug substances.

[0054] In certain situations the drug loaded polymer may be administeredin the gel state instead of as a solution. The gelation may be theresult of raising the temperature of a drug laden polymer solution toabove the gelation temperature of the polymer prior to administration,or may be caused by raising the concentration of the polymer in thesolution to above the saturation concentration at the temperature ofadministration, or it may be caused by additives to the polymer solutionwhich cause the solution to gel. In either event, the gel thus formedmay be administered parenterally, topically, transdermally,transmucosally, inhaled or inserted into a cavity such as by ocular,vaginal, buccal, transurethral, rectal, nasal, oral, pulmonary or auraladministration.

[0055] This invention is applicable to bioactive agents and drugs of alltypes including oligonucleotides, hormones, anticancer-agents. It offersan unusually effective way to deliver polypeptides and proteins. Manylabile peptide and protein drugs are amenable to formulation into theblock copolymers of the invention and can benefit from the reversethermal gelation process described herein. While not specificallylimited to the following, examples of pharmaceutically usefulpolypeptides and proteins may be selected from the group consisting oferythropoietin, oxytocin, vasopressin, adrenocorticotropic hormone,epidermal growth factor, platelet-derived growth factor (PDGF),prolactin, luliberin, luteinizing hormone releasing hormone (LHRH), LHRHagonists, LHRH antagonists, growth hormone (human, porcine, bovine,etc.), growth hormone releasing factor, insulin, somatostatin, glucagon,interleukin-2 (IL-2), interferon-, or , gastrin, tetragastrin,pentagastrin, urogastrone, secretin, calcitonin, enkephalins,endorphins, angiotensins, thyrotropin releasing hormone (TRH), tumornecrosis factor (TNF), nerve growth factor (NGF), granulocyte-colonystimulating factor (G-CSF), granulocyte macrophage-colony stimulatingfactor (GM-CSF), macrophage-colony stimulating factor (M-CSF),heparinase, bone morphogenic protein (BMP), hANP, glucagon-like peptide(GLP-1), interleukin-11 (IL-11), renin, bradykinin, bacitracins,polymyxins, colistins, tyrocidine, gramicidins, cyclosporins andsynthetic analogues, modifications and pharmacologically activefragments thereof, enzymes, cytokines, antibodies and vaccines.

[0056] The only limitation to the polypeptide or protein drug which maybe utilized is one of functionality. In some instances, thefunctionality or physical stability of polypeptides and proteins canalso be increased by the addition of various additives to aqueoussolutions or suspensions of the polypeptide or protein drug. Additives,such as polyols (including sugars), amino acids, surfactants, polymers,other proteins and certain salts may be used. These additives canreadily be incorporated into the triblock copolymer mixtures, which willthen undergo the reverse thermal gelation process of the presentinvention.

[0057] Developments in protein engineering may provide for thepossibility of increasing the stability of peptides or proteins. Whilesuch resulting engineered or modified proteins may be regarded as newentities to regulatory agents, those modifications do not alter theirsuitability for use in the present invention. One of the typicalexamples of modification is PEGylation, where the stability of thepolypeptide drugs can be significantly improved by covalentlyconjugating water-soluble polymers, such as polyethylene glycol, withthe polypeptide. Another example is modification of the amino acidsequence by addition, deletion or substitution of one or more amino acidresidues, at terminal and/or internal locations. Any such modifiedpolypeptide or protein with its improved stability or efficacy is withinthe scope of the present invention.

[0058] In addition to peptide or protein based drugs, other drugs fromall therapeutic and medically useful categories may be utilized. Thesedrugs are described in such well-known literature references as theMerck Index, the Physicians Desk Reference, and The PharmacologicalBasis of Therapeutics. A brief listing of specific agents is providedfor illustration purposes only, and shall not be deemed as limiting:anti-cancer agents such as mitomycin, bleomycin, BCNU, carboplatin,doxorubicin, daunorubicin, methotrexate, paclitaxel, taxotere,actinomycin D and camptothecin; antipsychotics such as olanzapine andziprasidone; antibacterials such as cefoxitin; anthelmintics such asivermectin; antivirals such as acyclovir; immunosuppressants such ascyclosporin A (cyclic polypeptide-type agent), steroids, andprostaglandins.

[0059] The following are examples that illustrate preferred embodimentsof the invention, but are intended as being representative only.

EXAMPLES Example 1 Synthesis of PLG-PEG-PLG Tri-Block Copolymers

[0060] The synthesis process utilized in this example is generallydisclosed in U.S. Pat. No. 6,004,573 and is hereby incorporated byreference. Poly(ethylene glycol)(PEG-1000; 29.4 gram) was weighed andtransferred into a 250-mL 3-neck round bottom flask. The flask wasattached to a vacuum line (1-2 torr) and immersed in an oil bath.Residual water was removed by raising the bath temperature to 155° C.for 3 hours. The flask was raised and allowed to cool to about 80° C.D,L-lactide(55.65 gram) and glycolide(14.95 gram) were weighed and addedto the flask. The headspace of the flask was replaced by dry nitrogenand heating resumed (130° C.) until the lactide and glycolide weremelted. Polymerization was initiated by addition of 0.02 g of stannousoctoate dropwise over a one minute time period. The reaction mixture washeated at 155° C. for an additional eight hours. Monomers were removedby vacuum (1-2 torr) for ca. 2 hours and the residue was dissolved incold water (4° C.). Water soluble oligomers and monomers were removed byprecipitation of the desired polymer in hot water (80° C.). Water wasremoved by freeze-drying. Molecular weight of the resulting triblockcopolymer was determined by gel permeation chromotograph and to be 4080Daltons. A 23% by weight aqueous solution of the polymer was soluble incold water and gelled at 13.1° C. as the temperature was raised.

Example 2

[0061] This example was done by following the procedure described inExample 1 but using 37.74 gram of poly(ethylene glycol)(PEG 1450), 49.1gram of D,L-lactide and 13.18 gram of glycolide in the synthesis.Molecular weight of the resulting triblock copolymer was determined bygel permeation chromotograph to be 4610 Daltons. A 23% by weight aqueoussolution of the synthesized polymer was soluble in cold water and gelledat 42.3° C.

Example 3

[0062] This example illustrates a method for making a triblock copolymermixtures with various selectable reverse thermal gelation temperaturesby mixing two pre-made individual triblock copolymer solutions.Tri-block copolymers prepared by the method described in Example 1(Component I) and Example 2(Component II) were dissolved in water toform 23% by weight solutions. These two solutions were mixed andgelation temperatures were measured. As illustrated in FIG. 1, thegelation temperature of the mixture of Component I and Component IItriblock copolymers rises from about 13° C. (100% Component I) when thecontent of Component II in the mixture increases, up to 42° C. when thecontent of Component II triblock copolymer reaches 100%.

Example 4

[0063] This example illustrates a method of preparing a triblockcopolymer mixture, having reverse thermal gelation properties, bysimultaneously synthesizing two different triblock copolymer componentsin one reaction vessel.

[0064] PEG 1450(22.74 gram) and PEG 1000(9.11 gram) were weighed andtransferred into a 250-mL 3-neck round bottom flask. The flask wasattached to a vacuum line (1-2 torr) and immersed in an oil bath.Residual water was removed by raising the bath temperature to 155° C.for 3 hours. Then, the flask was raised and allowed to cool to ca. 80°C. D,L-lactide(53.73 gram) and glycolide(14.1 gram) were weighed andadded to the flask. The headspace of the flask was replaced by drynitrogen and heating resumed (130° C.) until the lactide and glycolidewere completely melted. Polymerization was initiated by the addition of0.02 g of stannous octoate (dropwise over 1 minute). The reactionmixture was heated at 155° C. for an additional eight hours. Monomerswere removed by vacuum distillation(1-2 torr) for about 2 hours and theresidue was dissolved in cold water (4° C.). Water soluble oligomers andmonomers were removed by precipitation of the desired polymer in hotwater (80° C.). Water was removed by freeze-drying. Molecular weight ofthe resulting triblock copolymer was determined by gel permeationchromotograph and to be 4910 Daltons. The gelling temperature of thepolymer solution (23% by weight) was 22.1° C.

Example 5

[0065] This example illustrates a mixture of a triblock copolymersolution(23% by weight in water) having an extremely high viscosity anda triblock copolymer solution (23% by weight in water) having a lowviscosity.

[0066] A Component I tri-block copolymer was prepared according toExample 2 except that PEG1450 was used as B-polymer block, the weightratio of A-polymer block to B-polymer block was 2.6:1, and thelactide/glycolide mole ratio (L/G) in the A-polymer block was 3:1. A 23%by weigh aqueous solution of this Component I triblock copolymer wasvery viscous, with a honey-like consistency. Similarly, a Component IItri-block copolymer was prepared according to Example 2 except thatPEG900 was used as B-polymer block, the weight ratio of A-polymer blockB-polymer block was 0.8:1, and the lactide/glycolide mole ratio (L/G) inthe A-polymer block was 3:1. An aqueous solution of the Component IItri-block copolymer had a low viscosity and did not form a gel between0-100° C. at any concentration. Five mixtures of Components I andComponents II triblock copolymers were prepared with total polymercontent fixed at 23% by weight. The weight percent of Component IItriblock copolymer in the copolymer mixture was between 10 to 30%. Theviscosity profiles and gelation temperatures of the mixtures ofComponent I and Component II triblock copolymers are shown in FIG. 2. InFIG. 2, various concentrations of the Component II triblock copolymer inthe mixture are represented by the following symbols: 10% ( ), 15% (•),20% (^(••)), 25% ( ), 30% (<) and 100% (^(••)). The solid linerepresents the viscosity at which these copolymer mixture solutionsenter the gel phase at the gelation temperature indicated.

[0067] These data show that by mixing a low viscosity tri-blockcopolymer solution and a high viscosity triblock copolymer solution, theresulting mixture has a viscosity low enough to easily pass through a26-gauge hypodermic needle, and yet have a reverse thermal gelationtemperature of between 26 to 32° C.

Example 6 Drug Release

[0068] This example illustrates the drug release profile from triblockcopolymer mixture solutions of the present invention using zinc insulinas a model protein drug.

[0069] A Component I tri-block copolymer prepared in Example 1 wasdissolved in 10 mM HEPES buffer (pH 7.4) to form a 20% solution.Similarly, Component II solution(20%) 10 mM HEPES buffer (pH 7.4) wasprepared by dissolving a Component II tri-block copolymer prepared asdescribed in Example 1 except that PEG1450 was used as B-polymer block,the weight ratio of A A polymer block/B-polymer block was 2.15:1 and thelactide/glycolide(L/G) mole ratio of A-polymer block was 4:1. Twosolutions of these tri-block copolymers mixtures into 10 mM HEPESbuffer(pH 7.4, containing 50 mM EDTA) were also prepared by mixingComponent I tri-block copolymer/Component II tri-block copolymer byweight ratios of 1:1 and 1:3 respectively. The final copolymerconcentration in these solutions were 20% by weight.

[0070] Zinc insulin (7.5 mg) was suspended in 1.58 mL of HEPES buffer.Aliquots (20-μL) of the suspension were added to vials so that each vialhad 950 μg of the zinc insulin. To each vial was further added 100 μL ofone of the triblock copolymer solutions prepared as describe above. Thecontents of the vials were thoroughly mixed, and placed in an oven (37°C.) to cause gelling before 1 mL of pre-warmed (37° C.) dissolutionmedium (10 mM HEPES buffer (pH 7.4) containing 50 mM EDTA) was added.The vials were placed in a 37° C. oven and the dissolution medium wasreplaced periodically. Insulin in the dissolution medium was analyzed byHPLC. The results are presented in FIG. 3 with the following symbolsrepresenting various triblock copolymer solutions: 100% Component I ( ),100% Component II ( ), 1:3 w/w mixture of Component I and Component II(n), and 1:1 w/w mixture of Component I and Component II ( ).

[0071] These data show that the release rate of zinc-insulin could bemodulated to fit individual needs by mixing two different tri-blockcopolymer solutions, which provide a significant advancement in the art.

Example 7

[0072] Modulation of the erosion of triblock copolymer reverse thermalgelation systems by mixing two copolymer solutions.

[0073] Component I triblock copolymer prepared in the Example 1 andComponent II triblock copolymer prepared in the Example 2 were dissolvedindividually in water to form 23% by weight solutions. The Component IItriblock copolymer solution had a gel temperature of 42.3° C. andtherefore no gel was observable at 37° C. Three mixtures of aboveprepared Component I triblock copolymer solution and Component IItriblock copolymer solution were prepared at ratios of 3:1, 1:1 and 1:3respectively. 3 mL aliquots of these three solution mixtures were placedin 20 mL scintillation vials. After the gel was set (37° C., 1 minute),to each vial was added 5 mL of water (37° C.). Periodically, the waterlayer was collected and replaced. Water was removed by lyophilizationand the amount of polymer eroded was measured gravimetrically. Theresults presented in FIG. 4, with the following symbols representingvarious weight ratios of Component I and Component II in the mixtures:1:0 ( ), 3:1 ( ), 1:1 (^(••)), and 1:3 ( ). The results clearly showthat the duration of the gel phase could be modulated by mixing twopolymer solutions of different gelation temperatures.

[0074] While the invention has been described with reference to certainpreferred embodiments, those skilled in the art will appreciate thatvarious modifications, changes, omissions, and substitutions can be madewithout departing from the spirit of the invention. It isintended,therefore, that the invention be limited only by the scope ofthe following claims.

We claim:
 1. An aqueous biodegradable polymeric drug delivery composition possessing reverse thermal gelation properties comprised of an aqueous phase having uniformly contained therein: (a) an effective amount of a drug; and (b) a biodegradable polymeric system possessing reverse thermal gelation properties comprising a mixture of at least a Component I triblock copolymer and a Component II triblock copolymer, said triblock copolymers comprising biodegradable polyester A-polymer blocks and polyethylene glycol B-polymer blocks, wherein the B-polymer block of said Component I triblock copolymer has an average molecular weight of 900 to 2000 Daltons and the B-polymer block of said Component II triblock copolymer has an average molecular weight of 600 to 2000 Daltons, and wherein said Component I triblock copolymer has an average molecular weight of between 2500 to 8000 Daltons and said Component II triblock copolymer has an average molecular weight of between 800-7200 Daltons.
 2. The aqueous polymeric drug delivery composition according to claim 1 wherein the biodegradable polymeric system content of said composition is between about 3 and 50% by weight.
 3. The aqueous polymeric drug delivery composition according to claim 1 wherein the drug content of said composition is between about 0.0001 and 30% by weight.
 4. The aqueous polymeric drug delivery composition according to claim 1 wherein said drug is a polypeptide or protein, gene, hormone, anti-cancer or anti-cell proliferation agent.
 5. The aqueous polymeric drug delivery composition according to claim 4 wherein said polypeptide or protein is a member selected from the group consisting of oxytocin, vasopressin, adrenocorticotropic hormone, epidermal growth factor, platelet-derived growth factor (PDGF), prolactin, luliberin, luteinizing hormone releasing hormone (LHRH), LHRH agonists, LHRH antagonists, growth hormone (human, porcine, bovine, etc.), growth hormone releasing factor, insulin, erythropoietin, somatostatin, glucagon, interleukin-2 (IL-2), interferon-( , , or ), gastrin, tetragastrin, pentagastrin, urogastrone, secretin, calcitonin, enkephalins, endorphins, angiotensins, thyrotropin releasing hormone (TRH), tumor necrosis factor (TNF), nerve growth factor (NGF), granulocyte-colony stimulating factor (G-CSF), granulocyte macrophage-colony stimulating factor (GM-CSF), macrophage-colony stimulating factor (M-CSF), heparinase, bone morphogenic protein (BMP), hANP, glucagon-like peptide (GLP-1), interleukin-11 (IL-11), renin, bradykinin, bacitracins, polymyxins, colistins, tyrocidine, gramicidins, cyclosporins and synthetic analogues, modifications and pharmacologically active fragments thereof, enzymes, cytokines, antibodies, tissue fragments and vaccines.
 6. The aqueous polymeric drug delivery composition according to claim 4 wherein said polypeptide is a hepatitis vaccine, or synthetic analogue, modification or pharmacologically active fragment thereof.
 7. The aqueous polymeric drug delivery composition according to claim 4 wherein said hormone is a member selected from the group consisting of testosterone, estradiol, progesterone, prostaglandins, and synthetic analogues, modifications and pharmaceutical equivalents thereof.
 8. The aqueous polymeric drug delivery composition according to claim 7 wherein said anti-cancer agent is a member selected from the group consisting of mitomycin, bleomycin, BCNU, carboplatin, doxorubicin, daunorubicin, methotrexate, paclitaxel, taxotere, actinomycin D, camptothecin, synthetic analogues, modifications and pharmaceutical equivalents thereof.
 9. A method for the administration of a drug to a warm blooded animal in a controlled release form which comprises: (1) providing an aqueous biodegradable polymeric drug delivery composition possessing reverse thermal gelation properties comprised of an aqueous phase having uniformly contained therein: (a) an effective amount of a drug; and (b) a biodegradable polymeric system possessing reverse thermal gelation properties comprising a mixture of at least a Component I triblock copolymer and a Component II triblock copolymer, said triblock copolymers comprising biodegradable polyester A-polymer blocks and polyethylene glycol B-polymer blocks, wherein the B-polymer block of said Component I triblock copolymer has an average molecular weight of 900 to 2000 Daltons and the B-polymer block of said Component II triblock copolymer has an average molecular weight of 600 to 2000 Daltons, and wherein said Component I triblock copolymer has an average molecular weight of between 2500 to 8000 Daltons and said component II triblock copolymer has an average molecular weight of between 800-7200 Daltons; (2) maintaining said composition as a liquid at a temperature below the gelation temperature of said biodegradable polymeric system; and (3) administering said composition as a liquid to said warm blooded animal with the subsequent formation of a gel as the temperature of said composition is raised by the body temperature of said animal to be above the gelation temperature of the biodegradable polymeric system.
 10. The method according to claim 9 wherein said administration is by parenteral, ocular, topical, inhalation, transdermal, vaginal, buccal, transmucosal, transurethral, rectal, nasal, oral, pulmonary or aural means.
 11. The method according to claim 9 wherein the biodegradable polymeric system content of said composition is between about 3 and 50% by weight.
 12. The method according to claim 9 wherein the drug content of said composition is between about 0.0001 and 20% by weight.
 13. The method according to claim 9 wherein said drug administered is a polypeptide or protein, gene, hormone, anti-cancer or anti-cell proliferation agent.
 14. The method according to claim 13 wherein said polypeptide or protein is a member selected from the group consisting of erythropoietin, oxytocin, vasopressin, adrenocorticotropic hormone, epidermal growth factor, platelet-derived growth factor (PDGF), prolactin, luliberin, luteinizing hormone releasing hormone (LHRH), LHRH agonists, LHRH antagonists, growth hormone (human, porcine, bovine, etc.), growth hormone releasing factor, insulin, somatostatin, glucagon, interleukin-2 (IL-2), interferon-( , or ), gastrin, tetragastrin, pentagastrin, urogastrone, secretin, calcitonin, enkephalins, endorphins, angiotensins, thyrotropin releasing hormone (TRH), tumor necrosis factor (TNF), nerve growth factor (NGF), granulocyte-colony stimulating factor (G-CSF), granulocyte macrophage-colony stimulating factor (GM-CSF), macrophage-colony stimulating factor (M-CSF), heparinase, bone morphogenic protein (BMP), hANP, glucagon-like peptide (GLP-1), interleukin-11 (IL-11), renin, bradykinin, bacitracins, polymyxins, colistins, tyrocidine, gramicidins, cyclosporins and synthetic analogues, modifications and pharmacologically active fragments thereof, enzymes, cytokines, antibodies, tissue fragments and vaccines.
 15. The method according to claim 13 wherein said polypeptide is a hepatitis vaccine, synthetic analogue, modification or pharmacological active fragment thereof.
 16. The method according to claim 13 wherein said hormone is a member selected from the group consisting of testosterone, estradiol, progesterone, prostaglandins and synthetic analogues, modifications and pharmaceutical equivalents thereof.
 17. The method according to claim 13 wherein the anti-cancer agent selected from the group consisting of mitomycin, bleomycin, BCNU, carboplatin, doxorubicin, daunorubicin, methotrexate, paclitaxel, taxotere, actinomycin D, camptothecin, and synthetic analogues, modifications and pharmaceutically equivalents thereof.
 18. A process of preparing the biodegradable polymeric system of claim 1 comprising mixing the different types tri-block copolymer components before polymerization in one reaction pot and then polymerizing the triblock copolymer mixtures.
 19. The process of preparing the biodegradable polymeric system of claim 1 comprising synthesizing the different types of triblock copolymer components separately, and then mixing them into a mixture of the components.
 20. A biodegradable copolymer mixture made from the process of claim
 18. 21. A biodegradable copolymer mixture made from the process of claim
 19. 22. A method of adjusting the gelation properties of a biodegradable polymeric system without negatively affecting its gel quality by providing the biodegradable polymeric system of claim 1, wherein the gelation temperature of the system is adjusted by selecting proper individual biodegradable triblock copolymer components.
 23. The method according to claim 22, wherein the individual triblock polymer component can be selected based on at least one member of the group consisting of average molecular weights of A-polymer block, average molecular weights of B-polymer block, weight ratios of A-polymer block over B-polymer block, and types of triblock copolymer. 